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United States Patent |
5,292,795
|
Southwick
,   et al.
|
March 8, 1994
|
Very fine stable dispersions of block copolymers
Abstract
A stable polymer dispersion having an average particle size of no more than
1 micron comprising water, a surfactant and a polymer comprising at least
one conjugated diolefin block and/or vinyl aromatic hydrocarbon block and
at least one acrylic monomer block with the structure
##STR1##
wherein R.sub.1 is selected from the group consisting of hydrogen, phenyl
and alkyl radicals which are linear or branched and contain from 1 to 10
carbon atoms, and R.sub.2 is a cyclic or non-cyclic alkyl radical which is
branched at the first carbon atom, contains from 1 to 14 carbon atoms and
may contain a tertiary amine or an ether linkage.
Inventors:
|
Southwick; Jeffrey G. (Houston, TX);
Austgen, Jr.; David M. (Missouri City, TX)
|
Assignee:
|
Shell Oil Company (Houston, TX)
|
Appl. No.:
|
880946 |
Filed:
|
May 8, 1992 |
Current U.S. Class: |
524/562; 524/60; 524/68; 524/69; 526/329.2 |
Intern'l Class: |
C08L 057/00; C08L 095/00; C09D 005/02 |
Field of Search: |
524/562,68
526/329.2
|
References Cited
U.S. Patent Documents
2918391 | Dec., 1959 | Hornibrook | 524/562.
|
3432455 | Mar., 1969 | Rasicci | 260/29.
|
3480578 | Nov., 1969 | Witt | 260/237.
|
3506604 | Apr., 1970 | Benjamin | 260/29.
|
3513120 | May., 1970 | Pohlemann | 526/329.
|
3962197 | Jun., 1976 | Khanna | 526/329.
|
4001159 | Jan., 1977 | Imai et al. | 524/562.
|
4061833 | Dec., 1977 | Pelletier et al. | 524/562.
|
4199490 | Apr., 1980 | Kamiya et al. | 260/29.
|
4252852 | Feb., 1981 | Goth | 524/562.
|
4522972 | Jun., 1985 | Mondt et al. | 524/562.
|
4912184 | Mar., 1990 | Akasaki et al. | 526/329.
|
4937282 | Jan., 1990 | Pfoehler et al. | 524/820.
|
5212220 | May., 1993 | Gelles | 524/60.
|
Foreign Patent Documents |
0749266 | Oct., 1970 | BE | 524/562.
|
151456 | Oct., 1981 | DD.
| |
9015102 | Dec., 1990 | WO | 524/562.
|
475367 | Nov., 1976 | SU.
| |
Other References
U.S. application Ser. No. 853,645, filed Mar. 18, 1992, Gelles et al.
|
Primary Examiner: Michl; Paul R.
Assistant Examiner: Asinovsky; Olga
Attorney, Agent or Firm: Haas; Donald F.
Claims
We claim:
1. A stable polymer dispersion having an average particle size of no more
than 1 micron comprising water, a surfactant and a block copolymer having
a molecular weight of at least 1,000 comprising at least one polymer block
formed of a conjugated diene and/or a vinyl aromatic hydrocarbon and at
least one acrylic monomer block with the structure
##STR4##
wherein R.sub.1 is selected from the group consisting of hydrogen, phenyl
and alkyl radicals which are linear or branched and contain from 1 to 10
carbon atoms, and R.sub.2 is a cyclic or non-cyclic alkyl radical which is
branched at the first carbon atom, contains from 1 to 14 carbon atoms and
may contain a tertiary amine or an ether linkage.
2. The dispersion of claim 1 wherein the polymer comprises at least one
block of a conjugated diene and at least two blocks of a vinyl aromatic
hydrocarbon.
3. The dispersion of claim 2 wherein the acrylic monomer is an alkyl
methacrylate.
4. The dispersion of claim 3 wherein the alkyl methacrylate is tertiary
butyl methacrylate or methacrylate.
5. An adhesive comprising the dispersion of claim 1 and a tackifying resin.
6. A high tensile strength coating comprising the dispersion of claim 1
coalesced from water.
Description
BACKGROUND OF THE INVENTION
This invention relates to stable dispersions of polymers of conjugated
dienes. More specifically, it relates to such dispersions which contain
only low levels of surfactant.
It is known that polymers can be obtained by an anionic copolymerization of
a conjugated diene compound and, optionally, an alkenyl arene compound by
using an organic alkali metal initiator. Polymers have been produced which
comprise these different monomers such as polystyrene and butadiene or
isoprene or blocks thereof. These polymers may have configurations which
are linear, radial or star, i.e. many arms radiating from a central core.
In block copolymers, the proportion of thermoplastic blocks to elastomeric
blocks and the relative molecular weights of each of these blocks is
balanced to obtain a rubber having unique performance characteristics.
It has been found advantageous to prepare latexes of these polymers in
order to obtain products with different properties. For instance, either
protective coatings or adhesive films can be formed on surfaces by
evaporating water from latexes containing high molecular weight polymers.
Similar films of high molecular weight block copolymers are usually formed
by hot melt or solvent evaporation. These latexes are comprised of the
polymer, stabilizing surfactant(s), water and possibly coalescing
solvents.
It has proved difficult to make such dispersions without using relatively
high levels of surfactants, for instance 4 phr (parts per hundred rubber)
sodium dodecyl sulfate (SDS), because styrenic block copolymers are
incompatible with water and readily coagulate. In end uses such as
adhesives and moisture resistant coatings, that much surfactant cannot be
tolerated because adhesive tack is destroyed by the migration of
surfactant to the surface and surfactant allows a pathway for moisture
ingress in coatings. Adding functionality to the diene portion of the
molecule has been tried as a way to reduce the amount of surfactant
necessary to make dispersions of less than 1 micron particle size. For
instance, polymers described in U.S. Pat. No. 4,578,429 (carboxylic acid
or anhydride, such as maleic acid or anhydride, functionalized polymers)
have been tried but these still require 2 phr of surfactant to achieve 1
micron particle size.
We have found that stable polymer dispersions with an average particle size
of less than one micron can be made with the polymers of this invention.
Such fine polymer dispersions are desirable because they do not coagulate,
settle or cream quickly and fine particle sizes are known to be
advantageous in the coalescence of homogeneous films.
SUMMARY OF THE INVENTION
The present invention is a stable polymer dispersion having an average
particle size of no more than 1 micron which comprises water, a surfactant
and a polymer of a conjugated diolefin which contains an acrylic monomer
as part of the polymer backbone. The polymer used in the composition of
the present invention is comprised of at least one conjugated diolefin
block and/or vinyl aromatic hydrocarbon block and at least one acrylic
monomer block with the structure
##STR2##
where R.sub.1 is hydrogen, phenyl or an alkyl radical which is linear or
branched and has from 1 to 10 carbon atoms and R.sub.2 is an alkyl radical
which is branched at the first carbon atom, has from 1 to 14 carbon atoms,
may contain a tertiary amine or an ether linkage and may be a cyclic
hydrocarbon.
These polymers are functionalized in that they contain, in the polymer
backbone, acrylic, especially methacrylate, functionality. This provides
the polymer with strongly reactive and interactive chemical groups. In the
formula above, it is important that R.sub.2 be branched at the first
carbon because branching makes the monomer easier to polymerize. An
example is t-butyl which has the formula:
##STR3##
It is preferred that the acrylic monomers be alkyl methacrylates and the
preferred alkyl methacrylate is tertiary butyl methacrylate (TBMA).
The invention also comprises an adhesive containing such polymer and a
tackifying resin. Further, it is preferred that the tackifying resin
comprise from about 20 to about 400 parts by weight per 100 parts rubber
(phr). The preferred structure for use in this invention is a linear
styrene-isoprene-styrene-tertiary butyl methacrylate block copolymer.
DETAILED DESCRIPTION OF THE INVENTION
The polymers which may be used according to the present invention are
polymers of conjugated dienes and/or vinyl aromatic hydrocarbons and
acrylic monomers of the formula described above such as alkyl
methacrylates or derivatives of alkyl methacrylates such as hydrolyzed
alkyl methacrylates or anhydride derivatives thereof. Other suitable
acrylic monomers include acrylates, such as t-butyl acrylate; cyclic alkyl
methacrylates, such as 2,6-dimethylcyclohexyl methacrylate; and acrylates
in which the alkyl group contains an ether linkage, such as
tetrahydrofuran acrylate. Copolymers containing two or more conjugated
dienes are useful herein. Polymers containing no dienes are useful herein.
Copolymers of conjugated dienes and acrylic monomers with vinyl aromatic
monomers are preferred and both random and block copolymers thereof are
useful herein. The description which follows is described in terms of
block copolymers of conjugated dienes, alkyl methacrylates and vinyl
aromatic hydrocarbons but it is applicable also to the other polymers
described in this paragraph. This means that this invention encompasses
functionalized polymers which are not block copolymers but which
incorporate the functionality as described below.
The present invention encompasses polymers which are both high and low in
molecular weight, as well as in between. High molecular weight polymers
include those up to several million molecular weight as defined by gel
permeation chromatography (GPC) peak molecular weight of the main species.
Low molecular weight polymers include those of only 1000 molecular weight
or even less. In all cases these polymers contain both conjugated dienes
and acrylic monomers (alkyl methacrylates).
The preferred base polymers of the present invention are block copolymers
of conjugated dienes, acrylic monomers such as alkyl methacrylates or
their derivatives and vinyl aromatic hydrocarbons. Such block copolymers
may be multiblock copolymers of varying structures containing various
ratios of the monomers including those containing up to about 60% by
weight of vinyl aromatic hydrocarbon. At higher vinyl aromatic hydrocarbon
contents, the polymers are not elastomeric and would not be useful for
adhesives, sealants and flexible coatings. Thus, multiblock copolymers may
be utilized which are linear or radial, symmetric or asymmetric, and which
have structures represented by the formulae, ABAC, ABC, BC, BAC, CABAC,
CBC, (CB).sub.n X, (BC).sub.n X, (CB).sub.n XA.sub.m, (BC).sub.n XA.sub.m,
(CB).sub.n XB.sub.m, (BC).sub.n XB.sub.m, etc. where A is the vinyl
aromatic hydrocarbon, B is the diene, C is the acrylic monomer, X is a
coupling agent and n and m are integers from 1 to 50. These are just some
of the structures possible. Their finite number is not meant to limit the
scope of the invention. It is not necessary but B can be a polymer block
of a conjugated diene that has been hydrogenated. Hydrogenation of the
diene is preferred in applications requiring superior thermal stability.
It may be desirable to functionalize these block copolymers of methacrylate
and rubber. However, the routes to acid functionalizing involve exposing
the polymer to: (1) heat which eliminates isobutylene and water to form a
methacrylic anhydride which then forms methacrylic acid upon exposure to
water, or (2), hydrolysis of the ester group by heating
(70.degree.-90.degree. C.) a polymer solution in the presence of an acid
or acid catalyst. Both routes can possibly degrade and/or crosslink
unsaturated rubber if not done carefully. To circumvent this problem it is
preferred that the rubber block be hydrogenated.
The preferred polymers for use herein are block copolymers which contain a
block of conjugated diene, 2 blocks of a vinyl aromatic hydrocarbon and a
block of alkyl methacrylate because such polymers combine the physical
strength of styrenic block copolymers with the water dispersibility
characteristics of acrylics.
The block copolymers may be produced by any well known block polymerization
or copolymerization procedures including the well-known sequential
addition of monomer techniques, incremental addition of monomer technique
or coupling technique. As is well known in the block copolymer art,
tapered copolymer blocks can be incorporated in the multiblock copolymer
by copolymerizing a mixture of conjugated diene and vinyl aromatic
hydrocarbon monomers utilizing the difference in their copolymerization
reactivity rates. The manufacture of such polymers containing alkyl
methacrylates is described in U.S. Pat. No. 5,002,676 and copending
commonly assigned application Ser. No. 525,812, filed May 21, 1990, both
of which are herein incorporated by reference.
Conjugated dienes which may be utilized to prepare the polymers and
copolymers include those having from 4 to 8 carbon atoms and also include
1,3-butadiene,2-methyl-1,3-butadiene(isoprene),2,3-dimethyl-1,3-butadiene,
1,3-pentadiene, 1,3-hexadiene and the like. Mixtures of such conjugated
dienes may also be used. The preferred conjugated dienes are 1,3-butadiene
and isoprene.
Vinyl aromatic hydrocarbons which may be utilized to prepare copolymers
include styrene, o-methylstyrene, p-methylstyrene, p-tertbutylstyrene,
2,4-dimethylstyrene, alpha-methylstyrene, vinylnapthalene, vinylanthracene
and the like. The preferred vinyl aromatic hydrocarbon is styrene.
Alkyl methacrylates are preferred for use herein and those employed herein
include methacrylates wherein the alkyl group has up to 14 carbon atoms
inclusive. Derivatives of these polymers are also included herein, such
as, for example, polymers with partially or completely acidified
methacrylate groups, their anhydrides, their ionomers, their reaction
products with alcohols and amines, and the like. Derivatives of alkyl
methacrylates include methacrylic acid, methacrylic acid salts (for
example, zinc, sodium and quaternary ammonium salts) and anhydrides formed
between adjacent acid units by heating. This derivitization reaction can
be performed in situ with reactive esters such as t-butyl or 1,1-dimethyl
alkyl ester. Illustrative of such methacrylate esters are methyl
methacrylate, ethyl methacrylate, sec-butyl methacrylate, t-butyl
methacrylate, i-amyl methacrylate, hexyl methacrylate, decyl methacrylate
and dodecyl methacrylate. Largely because of ease of polymerization, the
preferred alkyl methacrylates are branched-butyl methacrylates, i.e.,
iso-butyl methacrylate and t-butyl methacrylate. The desired poly(alkyl
methacrylate) block is produced by directly polymerizing the corresponding
alkyl methacrylate monomer or alternatively the desired block is obtained
by polymerizing a more easily polymerizable methacrylate and subsequently
transesterifying the product to introduce the desired alkyl group. It is
also possible to copolymerize randomly or by sequential addition two or
more different acrylic monomers in the acrylic monomer block. Tertiary
butyl methacrylate (TBMA) is preferred because of ease of purification and
polymerization, and because it undergoes thermolysis at temperatures as
low as about 180.degree. C.
The present invention works with both unhydrogenated and hydrogenated
polymers. Hydrogenated ones are useful in certain circumstances. While
unhydrogenated diene polymers have a number of outstanding technical
advantages, one of their principal limitations lies in their sensitivity
to oxidation. This can be minimized by hydrogenating the copolymers,
especially in the diene blocks. The hydrogenation of these polymers and
copolymers may be carried out by a variety of well established processes
including hydrogenation in the presence of such catalysts as Raney Nickel,
noble metals such as platinum, palladium and the like and soluble
transition metal catalysts. Titanium biscyclopentadienyl catalysts may
also be used. Suitable hydrogenation processes which can be used are ones
wherein the diene-containing polymer or copolymer is dissolved in an inert
hydrocarbon diluent such as cyclohexane and hydrogenated by reaction with
hydrogen in the presence of a soluble hydrogenation catalyst. Such
processes are disclosed in U.S. Pat. Nos. 3,113,986, 4,226,952, 5,039,755
and Reissue 27,145, the disclosures of which are herein incorporated by
reference. The polymers are hydrogenated in such a manner as to produce
hydrogenated polymers having a residual unsaturation content in the
polydiene block of less than about 20%, and preferably as close to zero
percent as possible, of their original unsaturation content prior to
hydrogenation.
The molecular weights of these polymers may range from 1000 to 1,000,000,
preferably from 20,000 to 200,000. The vinyl aromatic hydrocarbon block
molecular weight generally ranges from 4,000 to 30,000 and the conjugated
diolefin block molecular weight generally ranges from 20,000 to 175,000.
The molecular weight of the acrylic monomer block should be in the range
from 142 to 30,000 because 142 represents the molecular weight of one unit
and molecular weights greater than 30,000 may form blocks that are so
large that their diffusion to the solid-aqueous interface is impaired.
The acrylic monomer content of the polymer is generally no more than about
20% but acrylic monomer contents of up to 70% are possible. Generally, the
acrylic monomer may be present in the polymer in an amount from about 1%
to about 20% because lower amounts will not provide the advantages of the
present invention and higher amounts are not advantageous from a cost
standpoint. All percentages expressed above are weight percentages based
on the total weight of the block copolymer.
The dispersions of the invention are made by starting with a solution of
the polymer in solvents such as cyclohexane or toluene or blends
containing cyclohexane or toluene and various cosolvents such as
methylethyl ketone, ethyl acetate, isopropyl alcohol, methyl
isobutylketone or isobutyl isobutyrate. In most instances, it will be
convenient to use the polymer cement directly from the polymerization
step. The polymer cement is emulsified by adding it to water which
contains from 0.1 to 10 weight percent at least one surfactant, preferably
less than 4 percent if the latex is to be used in an adhesive. It is
preferred that the polymer cement be added slowly, preferably in
increments, to a known volume of water containing the surfactant. This is
important because this procedure promotes the efficient preparation of
cement-in-water emulsions with small average drop sizes. Other preferred
elements of the process include using at least one anionic surfactant
because they are more efficient emulsifiers of the cement emulsions than
are cationic or nonionic surfactants. The final emulsion may contain up to
70 weight percent of the cement as the dispersed phase. This means that
although the emulsion is water continuous, it can contain as little as 30%
water. The solvent is removed from the emulsion by evaporating solvent
under vacuum and/or elevated temperature. Excess water may be removed at
the same time or later in order to concentrate the resulting latex to the
desired percent solids.
In a preferred embodiment of the present invention, a cosolvent is present
in the cement to assist in making very fine stable dispersions or
emulsions as described in copending commonly assigned application "Process
for Making Submicron Stable Latexes of Block Copolymers", filed
concurrently herewith. The cosolvent should be used in an amount from 5
weight percent to 50 weight percent of the total solvent in the cement. If
less than that amount is used, then there will be little effect on
emulsion droplet size and therefore, on latex particle size, and if more
is used, then the copolymer will be insufficiently solubilized in the
solvent/cosolvent blend and unstable emulsions will be formed. The
cosolvent should be chosen on the basis of its compatibility with or
affinity for the particular polymer which is to be emulsified. In the case
of the present polymers containing polar functional groups, the cosolvent
should be polar in nature because this effectively reduces the viscosity
of the cement at a given polymer level. Examples of cosolvents which are
polar in nature include isopropyl alcohol, methyl ethyl ketone, acetone,
isobutyl isobutyrate, ethyl acetate, methyl isobutyl ketone, n-butyl
alcohol and isopropyl acetate.
A variety of conventional emulsifiers or surfactants and mixtures of
emulsifiers or surfactants can be used to stabilize the polymer
dispersions. Such surfactants include anionic, nonionic and cationic
surfactants. Mixtures of nonionic surfactants with either anionics or
cationics are sometimes especially effective. The cationics most widely
used are salts of fatty amines, amido amines and imidazolines. The
anionics most widely used include sulfonates and sulfates with the general
formulas R-SO.sub.3 M and R-OSO.sub.3 M where R represents a hydrophobic
moiety and M represents an alkali metal. Examples include sodium dodecyl
sulfate, sodium lauryl sulfate, sodium salt of sulfated Neodol.RTM.
alcohols, sodium salts of sulfated Neodol.RTM. ethoxylates, sodium dodecyl
benzene sulfonate, sodium alkyl naphthalene sulfonate and sodium dioctyl
sulfosuccinate. Salts of sulfated alkyl-phenol ethoxylates are also
effective anionic emulsifiers. Examples of effective nonionic surfactant
emulsifiers include the family of alkyl-phenol ethoxylates represented by
the formula R-(OC.sub.2 H.sub.4).sub.n OH, where R is usually an octyl or
nonyl chain and n has a value from 1 to 35, preferably 4 to 15.
Specific examples of surfactants which are useful herein include
Neodol.RTM. 25-S, Neodol.RTM. 23-3S, Neodol.RTM. 23-9S, Neodol.RTM. 45-S,
Alipal.RTM. EP-110, Alipal.RTM. EP-120, Calsoft F90, Nekal.RTM. BA-77,
Emcol 4500, octyl phenol ethoxylates having 1 to 35 ethylene oxide groups
and nonyl phenol ethoxylates having 1 to 35 ethylene oxide groups such as
the Igepal.RTM. CA and CO series sold commercially by Rhone-Poulenc. Water
soluble nonionic block copolymers are also frequently used to stabilize
latexes. Examples of the latter include the Synperonic T range of
polypropylene oxide - polyethylene oxide block copolymers from ICI.
Surfactants should be used in as low an amount as possible because their
presence in adhesive films and protective coatings formed by drying the
water from this type of dispersion is detrimental. Surfactants reduce
adhesive tack and increase water penetration through protective coatings.
It is possible to use less than 2 phr of surfactant and achieve stable
dispersions of 1 micron or less because of the use of the polymers
described herein.
The process of the present invention produces polymer latexes which are
very stable and very fine, i.e., they have an average particle size of
less than one micron. Such polymer latexes are useful for formulating
water-based adhesives including pressure sensitive, contact and
construction mastic adhesives, especially where adhesion to polar surfaces
is desirable. Such latexes are also useful for producing water-based
coatings, especially with high tensile strength, with little or no
solvent.
Water based coatings and adhesives containing methacrylate groups can be
crosslinked by a variety of well-known crosslinking agents. The
methacrylic acid functional form of these polymers is preferred for the
crosslinking reactions. Examples of useful crosslinking agents include
Cymel.RTM. resins (based on either melamine or urea formaldehyde
chemistry); zirconium salt complexes such as Bacote 20 from Magnesium
Electron; zinc, aluminum, or chromium salts; polyaziridines, diols,
diamines and diisocyanates.
Particularly useful technologies for crosslinking these polymers coincident
with the evaporation and film formation from water are the use of Bacote
20 and zinc ammonium carbonate salts. These compounds can be mixed with
the polymer latex for an extended period of time and will only crosslink
chains during drying of the film. This behavior in the industry has been
termed good "pot life stability." Crosslinking of CBC, CABC, CABAC, or
coupled (BC).sub.n X Polymers can give coatings and adhesives with
enhanced high temperature and solvent resistant properties. Water-based
contact, mastic, pressure sensitive and laminating adhesives can be made
with dispersions of these polymers and their performance enhanced by
crosslinking the adhesives.
For adhesives, it is necessary to add an adhesion promoting or tackifying
resin that is compatible with the elastomeric isoprene block. A common
tackifying resin is a diene-olefin copolymer of piperylene and
2-methyl-2-butene having a softening point of about 95.degree. C. This
resin is available commercially under the tradename Wingtack 95 and is
prepared by the cationic polymerization of 60% piperylene, 10% isoprene,
5% cyclopentadiene, 15% 2-methyl-2-butene and about 10% dimer, as taught
in U.S. Pat. No. 3,577,398 incorporated by reference. Other tackifying
resins of the same general type may be employed in which the resinous
copolymer comprises 20-80 weight percent of piperylene and 80-20 weight
percent of 2-methyl-2-butene. The resins normally have softening points
(ring and ball) between about 800.degree. C. and about 1150.degree. C.
Other adhesion promoting resins which are also useful in the compositions
of this invention include hydrogenated rosins, esters of rosins,
polyterpenes, terpenephenol resins and polymerized mixed olefins. To
obtain good themo-oxidative and color stability, it is preferred that the
tackifying resin be a saturated resin, e.g. , a hydrogenated
dicyclopentadiene resin such as Escorez.RTM. 5000 series resin made by
Exxon or a hydrogenated polystyrene or polyalphamethylstyrene resin such
as Regalrez.RTM. resin made by Hercules.
The amount of adhesion promoting resin employed varies from 20 to 400 parts
by weight per hundred parts rubber (phr), preferably between 100 to 350
phr. The rubber referred to herein is the polymer used in the adhesive
composition.
The selection of the particular tackifying agent is, in large part,
dependent upon the specific block copolymer employed in the respective
adhesive composition. In the manufacture of disposable articles such as
diapers, sanitary napkins and bed pads, there is the additional
consideration of having a substantially white or clear adhesive
composition.
The adhesive composition of the instant invention may contain plasticizers,
such as rubber extending plasticizers, or compounding oils or liquid
resins. Rubber compounding oils are well-known in the art and include both
high saturates content oils and high aromatics content oils. Preferred
plasticizers are highly saturated oils, e.g. Tufflo.RTM. 6056 oil made by
Arco. The amounts of rubber compounding oil employed in the invention
composition can vary from 0 to 100 phr, and preferably between 0 to 60
phr.
Optional components of the present invention are stabilizers which inhibit
or retard heat degradation, oxidation, skin formation and color formation.
Stabilizers are typically added to the commercially available compounds in
order to protect the polymers against heat degradation and oxidation
during the preparation, use and high temperature storage of the adhesive
composition.
Additional stabilizers known in the art may also be incorporated into the
adhesive composition. These may be for protection during the life of the
article against, for example, oxygen, ozone and ultraviolet radiation.
However, these additional stabilizers should be compatible with the
essential stabilizers mentioned herein-above and their intended function
as taught herein.
EXAMPLE 1
Dispersions of various polymers in water were formed by dissolving the
polymer in cyclohexane, to a concentration of 20% polymer, emulsifying the
solution by sonification into water containing surfactant and/or polymeric
stabilizers and removing solvent with a rotovap. The stability of these
dispersions in water is a function of the chemistry of the polymer and the
stabilizing ability of the surfactant and/or stabilizers. Dispersions of
S-B-S, S-B-S-TBMA, and S-B-S-MA rubbers (7,000-35,000-7,000 MW;
7,000-35,000-7,000 MW and 9.1% TBMA, and 7,000-35,000-7,000 MW and 5.7%
MA, respectively) were prepared with 0.5, 1 and 2 phr sodium dodecyl
sulfate (SDS) by sonifying equal parts of aqueous surfactant solution with
the cyclohexane polymer solution and removing solvent with a rotovap. The
percent TBMA in these polymers was determined by Nuclear Magnetic
Resonance Spectroscopy. The results are shown in Table 1.
The S-B-S-TBMA and the S-B-S-MA polymers mentioned above, and their
hydrogenated analogs mentioned below, are in fact not synthetically
prepared with 100% yield. A competing side coupling reaction occurs during
the polymerization of the methacrylate block giving S-B-S-M-S-B-S (coupled
triblocks). Analysis of these polymers indicates that 70% of the material
is S-B-S-TBMA (or MA), and 30% of the material is the coupled triblock.
The same ratio of tetra block (S-EB-S-TBMA (or MA)) to coupled triblock is
observed for the hydrogenated version.
The S-B-S dispersions were unstable and formed large clumps of rubber
(coagulum) in the water. The S-B-S-TBMA and S-B-S-MA dispersions formed
stable dispersions of approximately 1.0 micron and 0.5 micron log normal
intensity average particle sizes, respectively, even with only 0.5 phr
surfactant. These stable dispersions, when coated in thin films, coalesced
into high tensile strength films at 150.degree. C. Adhesives and coatings
applications are increasingly looking to water based systems to reduce
pollution. For water based systems to adequately function as either an
adhesive or moisture resistant coating, only low levels of surfactant can
be tolerated. These results show that stable TBMA or MA containing
dispersions can be made with 0.5 phr of SDS. Prior experience has shown
that 2 phr SDS is necessary for stable maleic anhydride functionalized
block copolymers.
TABLE 1
______________________________________
Preparation of Dispersions
Film
Viscosity
Surfactant
Size Tensile
Solution cp phr micron
Stable?
psi
______________________________________
20% SBS 160 2 -- No --
1 -- No --
0.5 -- No --
20% 347 2 1.67 Yes 2285
SBS-TBMA 1 1.12 Yes 2990
0.5 0.97 Yes 1659
20% SBS-MA
1100 2 0.50 Yes 1878
1 0.56 Yes 2342
0.5 0.79 Yes 1092
______________________________________
EXAMPLE 2
Dispersions were also prepared with hydrogenated diene polymers. In this
case S-EB-S (7,000-35,000-7,000 MW), S-EB-S-TBMA (7,000-35,000-7,000 MW
with 5,000 MW TBMA block) and S-EB-S-MA (7,000-35,000-7,000 MW with 3,000
MW MA block) were dissolved in a solvent blend of 80 parts cyclohexane and
20 parts methyl ethyl ketone. Emulsions were prepared by sonifying equal
parts of these polymer solutions with aqueous surfactant solutions
containing varying amounts of sodium dodecyl sulfate. Emulsion particle
sizes were measured by dynamic light scattering.
Stabilities of the emulsions were rated after one month storage at room
temperature (nominally 23.degree. C). The data are compiled in Table 2.
Emulsions were rated unstable if solid rubber chunks were observed. The
partially stable emulsion had creamed, that is the organic phase had risen
to the top of the jar, but this cream could be easily re-dispersed into
the water by shaking in one's hand. Stable emulsions showed essentially no
changes during one month storage. We have found that emulsion stability
(before solvent removal) is predictive of the final dispersion stability
(after solvent removal by rotovap).
TABLE 2
______________________________________
Comparison of the Dispersibility in Water of Methacrylate
Modified SEBS Polymers with Corresponding SEBS Polymers
Viscosity
Surfactant
Size
Solution cp phr micron Stable?
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25% SEBS 2300 4 0.48 Yes
2 0.94 No
20% SEBS-TBMA
1500 1 1.5 Partially
0.6 -- No
20% SEBS-MA 2345 2 0.41 Yes
1.25 0.73 Yes
0.5 0.90 Yes
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The data demonstrate that incorporating a methacrylate block onto a
hydrogenated S-EB-S polymer gives polymers which can be dispersed in water
with less surfactant than the corresponding S-EB-S. The data show that a
5,000 MW tert-butyl methacrylate block improved water dispersibility, but
that conversion of this methacrylate ester to a methacrylic acid block
drastically improves dispersibility in water. Stable dispersions were
formed of S-EB-S-MA polymers with only 0.5 phr of sodium dodecyl sulfate.
This surfactant level is equivalent to surfactant concentrations normally
used in emulsion polymerizations. Low surfactant dispersions are required
in order to formulate adequately performing water-base adhesive systems.
EXAMPLE 3
A quantitative measure of stability was performed on polymers described in
Example 2. The S-EB-S polymer was dissolved in an 80/20 mixture of
cyclohexane/methyl ethyl ketone (MEK) to a concentration of 20%. A similar
20% solution of the S-EB-S-MA polymer was prepared in a 60/40 blend of
cyclohexane and MEK. These solvent blends have been found to be optimum
for each polymer. Emulsions were prepared via sonification against an
equal part of water containing 0.1% sodium dodecyl sulfate. The solvent
was removed with a rotovap. Final surfactant concentration based on
polymer was 0.5 phr. The initial solids content of the latexes was about
16 percent.
Table 3 shows the amount of material, expressed as a percentage of the
initially dispersed solids, which was retained when the dispersion was
poured through a 100 mesh stainless steel screen. These experiments were
performed at 1, 7 and 30 days after solvent was removed from the emulsion.
The SEBS-MA latex was stable. Only 0.1% of the material was retained on
the screen after one day and no additional material could be removed by
screening for 30 days. The corresponding S-EB-S latex was unstable. The
dispersed rubber particles were coagulating with time over 30 days. After
30 days, 7.5% of the original suspended solids had flocculated.
TABLE 3
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Comparison of Dispersion Stability of
SEBS and SEBS-MA in Water
Cumulative Coagulum (%) Removed
With 100 Mesh Stainless Steel Screen
One Day 7 Days 30 Days
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SEBS 2.7% 3.5% 7.5%
SEBS-MA 0.1% 0.1% 0.1%
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Dispersions stabilized with 0.5 phr SDS
Initial dispersed solids was 16%
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